Huntington's Disease (HD), also known as Huntington's Chorea, is a progressive disorder of motor, cognitive and psychiatric disturbances. The mean age of onset for this disease is age 35-44 years, although in about 10% of cases, onset occurs prior to age 21, and the average lifespan post-diagnosis of the disease is 15-18 years. Prevalence is about 3 to 7 among 100,000 people of western European descent.
Huntington's Disease is an example of a trinucleotide repeat expansion disorders were first characterized in the early 1990s (see Di Prospero and Fischbeck (2005) Nature Reviews Genetics 6:756-765). These disorders involve the localized expansion of unstable repeats of sets of three nucleotides and can result in loss of function of the gene in which the expanded repeat resides, a gain of toxic function, or both. Trinucleotide repeats can be located in any part of the gene, including non-coding and coding gene regions. Repeats located within the coding regions typically involve either a repeated glutamine encoding triplet (CAG) or an alanine encoding triplet (CGA). Expanded repeat regions within non-coding sequences can lead to aberrant expression of the gene while expanded repeats within coding regions (also known as codon reiteration disorders) may cause mis-folding and protein aggregation. The exact cause of the pathophysiology associated with the aberrant proteins is often not known. Typically, in the wild-type genes that are subject to trinucleotide expansion, these regions contain a variable number of repeat sequences in the normal population, but in the afflicted populations, the number of repeats can increase from a doubling to a log order increase in the number of repeats. In HD, repeats are inserted within the N terminal coding region of the large cytosolic protein Huntingtin (Htt). Normal Htt alleles contain 15-20 CAG repeats, while alleles containing 35 or more repeats can be considered potentially HD causing alleles and confer risk for developing the disease. Alleles containing 36-39 repeats are considered incompletely penetrant, and those individuals harboring those alleles may or may not develop the disease (or may develop symptoms later in life) while alleles containing 40 repeats or more are considered completely penetrant. In fact, no asymptomatic persons containing HD alleles with this many repeats have been reported. Those individuals with juvenile onset HD (<21 years of age) are often found to have 60 or more CAG repeats. In addition to an increase in CAG repeats, it has also been shown that HD can involve +1 and +2 frameshifts within the repeat sequences such that the region will encode a poly-serine polypeptide (encoded by AGC repeats in the case of a +1 frameshift) track rather than poly-glutamine (Davies and Rubinsztein (2006) Journal of Medical Genetics 43: 893-896).
In HD, the mutant Htt allele is usually inherited from one parent as a dominant trait. Any child born of a HD patient has a 50% chance of developing the disease if the other parent was not afflicted with the disorder. In some cases, a parent may have an intermediate HD allele and be asymptomatic while, due to repeat expansion, the child manifests the disease. In addition, the HD allele can also display a phenomenon known as anticipation wherein increasing severity or decreasing age of onset is observed over several generations due to the unstable nature of the repeat region during spermatogenesis.
Furthermore, trinucleotide expansion in Htt leads to neuronal loss in the medium spiny gamma-aminobutyric acid (GABA) projection neurons in the striatum, with neuronal loss also occurring in the neocortex. Medium spiny neurons (MSN) that contain enkephalin and that project to the external globus pallidum (in the so-called “indirect pathway”) are more involved than neurons that contain substance P and project to the internal globus pallidum (in the “direct” pathway), however both types of MSN are affected. MSNs in HD display transcriptional dysregulation along with other abnormal alterations (abnormal aggregations and inclusions of htt, bioenergetic defects, neurotophin deficiency, disorders of axonal transport and exictotoxicity). The mechanism for the transcriptomic effects may be related to changes in activities of soluble DNA-binding transcription factors, abnormalities of chromatin biochemistry and organization, and aggregate-driven nuclear transcription factor sequestration (see Runne et al (2008) J Neurosci 28(39):9723-9731).
Other brain areas greatly affected in people with Huntington's disease include the substantia nigra, cortical layers 3, 5, and 6, the CA1 region of the hippocampus, the angular gyrus in the parietal lobe, Purkinje cells of the cerebellum, lateral tuberal nuclei of the hypothalamus, and the centromedialparafascicular complex of the thalamus (Walker (2007) Lancet 369:218-228).
The role of the normal Htt protein is poorly understood, but it may be involved in neurogenesis, apoptotic cell death, and vesicle trafficking. In addition, there is evidence that wild-type Htt stimulates the production of brain-derived neurotrophic factor (BDNF), a pro-survival factor for the striatal neurons. It has been shown that progression of HD correlates with a decrease in BDNF expression in mouse models of HD (Zuccato et al (2005) Pharmacological Research 52(2): 133-139), and that delivery of either BDNF or glial cell line-derived neurotrophic factor (GDNF) via adeno-associated viral (AAV) vector-mediated gene delivery may protect striatal neurons in mouse models of HD (Kells et al, (2004) Molecular Therapy 9(5): 682-688).
Treatment options for HD are currently very limited. Some potential methodologies designed to prevent the toxicities associated with protein aggregation that occurs through the extended poly-glutamine tract such as overexpression of chaperonins or induction of the heat shock response with the compound geldanamycin have shown a reduction in these toxicities in in vitro models. Other treatments target the role of apoptosis in the clinical manifestations of the disease. For example, slowing of disease symptoms has been shown via blockage of caspase activity in animal models in the offspring of a pairing of mice where one parent contained a HD allele and the other parent had a dominant negative allele for caspase 1. Additionally, cleavage of mutant HD Htt by caspase may play a role in the pathogenicity of the disease. Transgenic mice carrying caspase-6 resistant mutant Htt were found to maintain normal neuronal function and did not develop striatal neurodegeneration as compared to mice carrying a non-caspase resistant mutant Htt allele (see Graham et al (2006) Cell 125: 1179-1191). Molecules which target members of the apoptotic pathway have also been shown to have a slowing effect on symptomology. For example, the compounds zVAD-fmk and minocycline, both of which inhibit caspase activity, have been shown to slow disease manifestation in mice. The drug remacemide has also been used in small HD human trials because the compound was thought to prevent the binding of the mutant Htt to the NDMA receptor to prevent the exertion of toxic effects on the nerve cell. However, no statistically significant improvements were observed in neuron function in these trials. In addition, the Huntington Study Group conducted a randomized, double-blind study using Coenzyme Q. Although a trend towards slower disease progression among patients that were treated with coenzyme Q10 was observed, there was no significant change in the rate of decline of total functional capacity. (Di Prospero and Fischbeck, ibid). U.S. Patent Publications 2011/0082093 and 20130253040 disclose nucleases targeted to Htt.
Various methods and compositions for targeted cleavage of genomic DNA have been described. Such targeted cleavage events can be used, for example, to induce targeted mutagenesis, induce targeted deletions of cellular DNA sequences, and facilitate targeted recombination at a predetermined chromosomal locus. See, e.g., U.S. Pat. Nos. 8,623,618; 8,034,598; 8,586,526; 6,534,261; 6,599,692; 6,503,717; 6,689,558; 7,067,317; 7,262,054; 7,888,121; 7,972,854; 7,914,796; 7,951,925; 8,110,379; 8,409,861; U.S. Patent Publications 20030232410; 20050208489; 20050026157; 20060063231; 20080159996; 201000218264; 20120017290; 20110265198; 20130137104; 20130122591; 20130177983 and 20130177960 and U.S. application Ser. No. 14/278,903, the disclosures of which are incorporated by reference in their entireties for all purposes. These methods often involve the use of engineered cleavage systems to induce a double strand break (DSB) or a nick in a target DNA sequence such that repair of the break by an error born process such as non-homologous end joining (NHEJ) or repair using a repair template (homology directed repair or HDR) can result in the knock out of a gene or the insertion of a sequence of interest (targeted integration). This technique can also be used to introduce site specific changes in the genome sequence through use of a donor oligonucleotide, including the introduction of specific deletions of genomic regions, or of specific point mutations or localized alterations (also known as gene correction). Cleavage can occur through the use of specific nucleases such as engineered zinc finger nucleases (ZFN), transcription-activator like effector nucleases (TALENs), or using the CRISPR/Cas system with an engineered crRNA/tracr RNA (‘single guide RNA’) to guide specific cleavage. Further, targeted nucleases are being developed based on the Argonaute system (e.g., from T. thermophilus, known as ‘TtAgo’, see Swarts et al (2014) Nature 507(7491): 258-261), which also may have the potential for uses in genome editing and gene therapy.
Engineered fusion proteins have also been developed for modulating expression of a targeted gene. Such proteins can be used for example to enhance or repress expression of a desired gene (see e.g. U.S. Pat. Nos. 6,534,261; 6,607,882; 6,824,978; 6,933,113; 7,013,219; 7,220,719; 8,268,618; 7,985,778; 8,586,526; U.S. Patent Application 20120294838 the disclosures of which are incorporated by reference in their entireties for all purposes).
Thus, there remains a need for compositions and methods that can draw from these promising techniques for the treatment and prevention of Huntington's Disease.